Devices of this type are normally used for softening the movement, brought about by spring means, of sliding drawers, such as some types of ash-tray in the dashboard of automotive vehicles, or of hinged doors, of the type used for example in cassette recorders. To this end, such devices comprise a rotor partially received within a chamber of a casing filled with a viscous fluid. The chamber is closed in a sealed manner by a cover, through which emerges a shaft of the rotor. In the majority of the devices on the market, the cover is fixed by welding to a shoulder surface of the casing.
During the mounting of the components on the casing, compression of the viscous fluid occurs, which fluid tends to emerge through the interstice between the cover and the wall of the casing which encloses the rotation chamber. If the viscous fluid succeeds in soiling the shoulder surface onto which the cover is to be welded, this fact may have a negative influence on the quality of the weld, and therefore the seal of the cover. For this reason, the amount of viscous liquid is generally limited compared with that which could theoretically be contained within the chamber. This, however, results in the presence, within the assembled and welded device, of air sacs which may adversely affect the performance of the device.